1. Field:
The instant invention relates to solid oxide fuel cells and particularly to ceramic interconnect materials having good electrical properties.
2. State of the Art:
Solid oxide fuel cells (SOFC's) are structured to convert the energy of combustion directly to electrical energy. Low molecular weight, residue-free gases, especially natural gas, carbon monoxide, hydrogen and other clean-burning gases, are employed as fuels. A solid electrolyte, e.g. ZrO.sub.2, which rapidly transports oxygen ions is an essential component in SOFC's.
Typical SOFC's are illustrated in the following U.S. patents:
U.S. Pat. No. 4,476,198 Ackerman, et al.
U.S. Pat. No. 4,816,036 Kotchick
U.S. Pat. No. 4,476,196 Poeppel, et al.
The fuel cell operation is shown schematically in FIG. A, wherein oxygen is introduced at the cathode, dissociates to form oxygen ions by picking up electrons from the external circuit. The oxygen ions flow through the electrolyte (which is at an elevated temperature .about.700.degree. C. or more) to combine with hydrogen, for example, in a combustion reaction (exothermic). The electrochemical heat of reaction and the internal resistance maintains the fuel cell at an efficient operating temperature, i.e., one at which the ceramic electrolyte, typically ZrO.sub.2, is an efficient transporter of oxygen ions. The combustion reaction (half cell reaction at the anode) is as follows: EQU O.sup.= +H.sub.2 .fwdarw.H.sub.2 O+2e.sup.-
The electrons freed by this reaction are available as electrical energy to perform useful work. The circuit must be complete so that the electrons are available at the cathode-electrolyte interface to participate in the dissociation of oxygen molecules into oxygen ions, to wit: EQU O.sub.2 +4e.sup.- .fwdarw.2O.sup.=
Ceramic interconnect devices interconnect one cell to another electrically and act as channels for both the gaseous fuel and oxygen, as illustrated in FIG. B. While FIG. B shows only two cells connected by a single interconnect, it is typical that a plurality of interconnects are used to form a "stack" of cells, thus serially connecting one cell to another from an electrical standpoint.
The interconnect must be a good conductor of electricity, have a coefficient of thermal expansion (CTE) which closely matches the electrolyte, e.g. zirconia, and be thermodynamically stable simultaneously at high oxygen partial pressures in oxygen or air and low oxygen partial pressures in the fuel gas at cell operating temperatures. Many materials may satisfy one or two of these requirements, but the lack of effective, long lasting interconnects has thus far retarded the development of a commercially usable fuel cell, such as those made of lanthanum strontium chromite (LSC).